"It's all about the boost," say the technicians at ABS Power Brake Company. Brake boosters and master cylinders-they're not the most glamorous parts of a brake system, especially when compared to the shiny drilled and slotted rotors equipped with multi-piston calipers. Be that as it may, these two brake components are the heart of any braking system. Seldom do people give these items a thought until they go bad or the brakes are upgraded. You can have monster sized rotors, and calipers with more pistons than an octopus has legs, but it means nothing unless the booster can produce enough line pressure in relation to the weight of the vehicle to make it all work.
Boosters
As vehicle technology has progressed through the decades, so have the various braking systems. The user now has options such as a dual-piston master cylinder, vacuum boost brakes, dual-diaphragm vacuum boost brakes, hydroboost brakes, electric hydroboost brakes, and varying in size from 7-11 inches
Let's focus on the typical brake set up on most street-driven Chevrolets. That would be vacuum-assisted brakes. SA Design Books has published an information packed book entitled High Performance Brake Systems by James Walker Jr. Here is a quote from Mr. Walker about vacuum boosters. "The vacuum booster provides gain by dividing the volume inside can into two chamber separated by a semi-flexible diaphragm. While the engine is running, a vacuum line from the engine's intake manifold draws nearly all of the air out of the booster, creating a vacuum on both sides of the diaphragm. Because there is equal pressure on both sides of the diaphragm, it remains in place with out doing anything at all. When the brakes are applied the vacuum booster works its magic. The incoming force from the brake pedal out-put rod cracks open a small labyrinth-like passageway in the booster internal power valve. This then allows a small amount air to enter the rearmost chamber of the booster, creating a pressure difference between the two chambers separated by the diaphragm.
Brake boosters can range in...
Brake boosters can range in size from the small 8-inch single boosters to the large diameter 11-inch single booster and everything else in between. Hood space and valve cover clearance issues (along with budget) can play a big role in which type of booster ultimately ends up in your vehicle.
"Because the diaphragm is not locked in place, it moves slightly away from the high pressure chamber to the low pressure chamber, and as it moves it drags the booster out put rod along with it. More brake pedal output force allows more air into the rearmost chamber, resulting in more pressure difference and consequently more movement ... For this reason larger boosters generate higher output forces."
In the average American mind, bigger is better and while that holds true for braking and boosters, it's not always possible due to under hood clearance, valve cover clearance and even cam size. Ask any Tri-Five Chevy owner. Despite all that space under the hood there is simply not enough room for a large diameter booster. (See our sidebar on hydro-boosters)
Master Cylinders
Doubtless, all reading this are in love with their classic Chevrolet vehicles. Some may still be using a single piston master cylinder that came with the car. That's nice if the vehicle is a restored show car or museum piece and sees very little street operation. There are more than a few of us (myself included) who have experienced the pedal to the floor syndrome of a single-piston master cylinder when it decides to fail. So much for the looks of the classic vehicle should the OE brakes fail. An experience like that can scare many into going to church.
 Externally brake boosters...  Externally brake boosters look no more complex than a soda pop can, internally it is a different story. Take a look inside this cut away dual diaphragm booster. Booster rebuilding is not a do it yourself type of job. |  Now take a look inside this...  Now take a look inside this single-diaphragm booster. It may appear less complex than the larger dual diaphragm booster, but it functions in exactly the same manner. Opening up a booster can have adverse affects on it future functionality, so resist the urge and keep them closed. |  The red arrow indicates piston...  The red arrow indicates piston movement when brakes are applied. With a cutaway dual circuit master cylinder it is easy to see both the primary and secondary pistons and their direction of travel. |